Flashback blood collection needle with needle shield

ABSTRACT

The needle assembly is provided with the housing, an IV cannula for accessing a blood vessel and a non-patient cannula for communication with an evacuated tube. A flashback chamber is provided in the housing at or near the distal end of the housing. A shield is hinged to a portion of the housing proximally of the entrance to the flashback chamber. As a result, good visibility of the flashback chamber is achieved. After use, the shield can be rotated into a closed position for surrounding the IV cannula and avoiding accidental needle sticks.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] A shieldable blood collection needle assembly with a flashbackchamber for providing confirmation of venous entry.

[0003] 2. Description of Related Art

[0004] Needle assemblies are used for collecting specimens of fluid,such as blood, from a patient. Some such needle assemblies are intendedfor use with an evacuated tube, such as tubes sold under the trademarkVACUTAINER™ by Becton Dickinson. Needle assemblies for use with anevacuated fluid collection tube include a needle hub with a proximalend, a distal end and a passage extending between the ends. The needleassembly further includes at least one needle cannula mounted to theneedle hub. The needle cannula includes a sharply pointed distal endthat projects distally beyond the needle hub, a proximal end thatprojects proximally beyond the needle hub and a lumen that providescommunication between the opposed ends of the needle cannula. Someneedle assemblies include separate proximal and distal cannulas and relyupon a portion of the needle hub to provide communication between thelumens of the respective cannulas. The distal end of the needle cannulatypically is beveled to a tip that is sufficiently sharp for piercingthe skin of the patient and accessing the vein or other source of fluidthat is to be collected. The proximal end of the needle cannula isconfigured for piercing the rubber stopper on an evacuated tube. Theproximal end of the needle cannula typically is covered by a needlepierceable resealable multi-sample sleeve. The sleeve is compressed bythe rubber stopper of the evacuated tube and punctured by the proximalend of the needle cannula as the proximal end of the needle cannula isurged into communication with the evacuated tube.

[0005] One problem with collecting fluid from a patient relates touncertainty during attempts to access the proper source of fluid in thepatient. For example, a blood collection procedure typically requiresthe phlebotomist to visually locate a vein and then to enter the veinwith the distal end of the needle cannula. The phlebotomist may not havea positive indication of venous entry at this stage. An evacuated tubethen is placed in communication with the proximal end of the needlecannula once the phlebotomist is reasonably certain that the targetedblood vessel has been entered. The low pressure within the evacuatedtube then cooperates with the higher pressure of the blood to generate aflow of blood into the tube. The flow of blood into the tube may be thefirst positive indication to the phlebotomist that the targeted bloodvessel has been accessed. The initial flow of blood along the longneedle cannula and into the evacuated tube may take a relatively longtime based on several factors, including the relative pressure levelsand the length and cross-sectional dimensions of the needle cannula. Thephlebotomist may interpret the absence of an immediate flow of blood asbeing a sign that the blood vessel was not targeted properly, when infact the absence of a visible blood flow in the evacuated tube may beattributable to pressure and fluid flow characteristics. Thus, thephlebotomist may unnecessarily withdraw the needle and start the bloodcollection procedure again. As a result, time is wasted and trauma forthe patient is increased. In view of the above problems, many fluidcollection needle assemblies are provided with a flashback chamber thatcommunicates with the needle cannula. The flashback chamber typically isformed at least partly from a transparent or translucent material and isintended to receive a portion of the blood flow shortly after a vein hasbeen accessed properly.

[0006] The blood collection needle assembly is withdrawn from thepatient after a suitable number of samples have been collected. The usedneedle assembly then is discarded. The medical profession is well awarethat accidental sticks with a used needle cannula can transmit disease.Accordingly, it is desirable to shield the used needle cannulaimmediately after the needle cannula has been withdrawn from thepatient. Shields have taken many different forms. For example, someshields telescope in a distal to proximal direction over the needlecannula and frictionally engage the needle hub. Shields of this typecreate the risk of an accidental needle stick during shielding if theshield is misaligned with the needle cannula. Some shields of thisgeneral type are provided with enlarged collars that are intended tominimize the risk of accidental sticks during shielding. However,shields of this general type are not preferred. Other shields aretelescoped over the needle hub and can be moved distally over the needlecannula to effect shielding. Shields of this general type are safe andeffective and are used in many situations. However, shields of this typecan interfere with the normal usage of some medical implements. Othershields are hingedly mounted to or near the needle hub and can berotated from an open position where the needle cannula is exposed to aclosed position where the needle cannula is shielded. Shields of thistype also work very well and are widely accepted. However, the existenceof a hinged shield on a fluid collection needle assembly with aflashback chamber is intuitively problematic in view of the complexitiesof providing both shielding and an unobstructed view of the flashbackchamber.

SUMMARY OF THE INVENTION

[0007] The subject invention is directed to a shieldable fluidcollection needle assembly. The needle assembly includes a housing witha proximal end wall, a distal end wall and an external sidewallextending between the proximal and distal end walls such that a chamberis defined between the end walls and the external sidewall. The externalsidewall may be formed unitarily with at least one of the end walls.Additionally, at least a portion of the external sidewall is formed froma transparent or translucent material so that the interior of thechamber within the housing is visible. A proximal passage extendsthrough the proximal end wall of the housing and a distal passageextends through the distal end wall of the housing. The proximal anddistal passages maybe axially aligned with one another. The housing mayfurther include an internal sidewall extending proximally from thedistal end wall and surrounding the entry of the distal passage into thechamber. The internal sidewall also is a transparent or translucentmaterial.

[0008] The needle assembly further includes a distal cannula mounted inthe distal passage of the housing and projecting distally beyond thehousing. The distal cannula includes a distal end spaced externally fromthe housing. The distal end may be beveled to a tip that is sufficientlysharp to pierce skin of a patient. The distal cannula includes a lumenthat extends from the distal end of the distal cannula and communicateswith the chamber of the housing.

[0009] The needle assembly further includes a proximal cannula mountedin the proximal passage of the housing. The proximal cannula includes aproximal end that projects proximally beyond the housing. Additionally,the proximal cannula includes a lumen that extends from the proximal endand into communication with the chamber. The proximal cannula alsoincludes a distal end that may extend into the chamber so that theproximal end of the distal cannula and the distal end of the proximalcannula substantially align with one another and are slightly spacedfrom one another. In an alternate embodiment, distal and proximalcannula may be integral with one another and may include a transverseslot or aperture to provide communication between the lumens of thedistal and proximal cannulas with the chamber in the housing.

[0010] The proximal end of the distal cannula preferably is at or nearthe distal end wall of the housing. Additionally, the distal end of theproximal cannula preferably extends through a major portion of thechamber of the housing and to a location near the distal end wall of thehousing. Thus, both the distal and proximal cannulas preferablycommunicate with the chamber of the needle assembly at a location nearthe distal end wall of the housing. As a result, the first indication offluid flow through the distal cannula will occur at a location at orsubstantially adjacent the distal end wall of the housing. In apreferred embodiment, the housing may have the internal sidewalldescribed above and the distal end of the proximal cannula may projectaxially into the cylindrical space bounded by the internal sidewall.

[0011] The needle assembly of the subject invention further includes ashield that may be hinged to a location on the housing proximally of thelocation where the first indication of flashback occurs. The shieldinitially is in an open position where the shield is spaced from thedistal cannula and the chamber. Hence, the shield does not impede usageof the distal cannula or visual observation of the chamber when theshield is in its open position. However, the shield can be rotated fromthe open position to a closed position where the shield surrounds thedistal cannula. The shield and/or the housing may include at least onelocking element for locking the shield in the closed position. Thelocking means can include a resiliently deflectable cannula lockconfigured for engaging the needle cannula when the shield is in theclosed position. The shield may further include locks for engagingstructure on the housing when the shield is in the closed position. Theengagement between structure on the shield and structure on the housingmay function to lock the shield in the closed position. Alternatively,primary locking of the shield in the closed position may be achieved bythe cannula lock, and the engagement between the shield and the housingmay be provided primarily for audible and tactile indication of completeshielding.

DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is an exploded perspective view of a needle assembly inaccordance with the invention.

[0013]FIG. 2 is a longitudinal cross-sectional view of the needleassembly prior to use.

[0014]FIG. 3 is a side elevational view of the needle assembly afterbeing placed in communication with a blood vessel, but before beingplaced in communication with an evacuated tube.

[0015]FIG. 4 is a longitudinal cross-sectional view of the needleassembly during use and after being placed in communication with anevacuated tube.

[0016]FIG. 5 is a side elevational view of the needle assembly aftercomplete shielding.

DETAILED DESCRIPTION

[0017] A needle assembly in accordance with the subject invention isidentified generally by the numeral 10 in FIGS. 1-5. Needle assembly 10includes a housing 12 with a proximal end 14, a distal end 16 and agenerally frustum-shaped outer sidewall 18 extending between the ends.Outer sidewall 18 is formed from a transparent or translucent plasticmaterial and defines a chamber 20 within housing 12 between proximal end14 and distal end 16. A proximal passage (not shown) extends throughproximal end 14 of housing 12 and communicates with chamber 20. A distalpassage 22 extends through distal end 16 of housing 12 and alsocommunicates with chamber 20. Housing 12 further includes a cylindricalinner sidewall 24 that extends from distal end 16 toward proximal end14. Inner sidewall 24 is substantially concentrically disposed withinouter sidewall 18 and is substantially concentrically generated aboutdistal passage 22. Inner sidewall 24 also is formed from a transparentor translucent plastic material. Hence, a fluid material in a portion ofchamber 20 bounded by inner sidewall 24 is readily visible fromlocations externally of outer sidewall 18. Thus, inner sidewall 24 inthis embodiment defines a flashback chamber 26, and fluid collected inflashback chamber 26 can be observed from locations externally of needleassembly 10. Chamber 26 communicates with chamber 20, and hence chamber20 may also contribute to an indication of flashback.

[0018] Needle assembly 10 further includes an IV cannula 30 with aproximal end 32, a distal end 34 and a lumen 36 extending between theends. Distal end 34 of IV cannula 30 is disposed externally of housing12 and is beveled to a sufficiently sharp tip for piercing skin andtissue of a patient. IV cannula 30 is mounted in distal passage 22 ofhousing 12 so that proximal end 32 of IV cannula 30 is substantiallyadjacent distal end 16 of housing 12. Thus, lumen 36 through IV cannula30 communicates with flashback chamber 26 at a location substantiallyadjacent distal end 16 of housing 12.

[0019] Needle assembly 10 further includes a non-patient (NP) cannula 40mounted in the proximal passage at proximal end 14 of housing 12. NPcannula 40 includes a proximal end 42 disposed externally of housing 12,a distal end 44 disposed in flashback chamber 26 and a lumen (not shown)extending between the ends. The proximal end 42 is beveled to asufficiently sharp tip for piercing a rubber stopper of an evacuatedtube as explained further herein. The lumen through NP cannula 40 isaligned substantially axially with lumen 36 through IV cannula 30.Distal end 44 of NP cannula 40 preferably is spaced only a shortdistance from proximal end 32 of IV cannula 30, such as 0.5 mm-1.2 mm.

[0020] Needle assembly 10 further includes a sleeve 46 mounted overportions of NP cannula 40 that are disposed externally of housing 12.Sleeve 46 is mounted to proximal end 14 of housing 12, and is formedfrom a material that is substantially impervious to liquid and fluid.However, sleeve 46 also is formed from a material that is readilypierceable by proximal end 42 of NP cannula 40 and that is resilientlyresealable. Thus, the rubber stopper of an evacuated tube can be urgedagainst sleeve 46 and will cause sleeve 46 to collapse distally. As aresult, the beveled tip at proximal end 42 of NP cannula 40 will piercethrough sleeve 46 and through the rubber stopper on the evacuated tube.

[0021] IV cannula 30, NP cannula 40 and housing 12 cooperate to providean early indication of venous entry due to the flashback of blood inchamber 26 and/or chamber 20 of housing 12. However, other flashbackdesigns can be incorporated into the needle assembly of the subjectinvention. These other designs include arrangements were blood firstfills the equivalent of sleeve 46 on needle assembly 10 and then flowsinto a flashback chamber. Still other designs include the use of avented plug. Flashback chamber designs that can be incorporated into thesubject invention are shown, for example, in U.S. Pat. No. 5,542,932,U.S. Pat. No. 5,824,001 and others.

[0022] Needle assembly 10 further includes a collar 50 securely mountedaround proximal portions of housing 12. Collar 50 may be securedmechanically in position on housing 12, may be bonded to housing 12 ormay be molded as part of housing 12. Collar 50 includes a proximal end52 substantially aligned with proximal portions of chamber 20 and adistal end 54 disposed proximally of proximal end 32 of IV cannula 30and near proximal end of inner sidewall 24. Accordingly, collar 50 willnot impair the ability to observe regions of flashback chamber 26adjacent proximal end 32 of IV cannula 30. Distal end 54 of collar 50 isprovided with a C-shaped hook 56 generated about an axis that extendstransverse to IV cannula 30 and NP cannula 40. Collar 50 furtherincludes a chevron-shaped projection 58 at a location diametricallyopposite hook 56. Collar 50 preferably is secured to housing 12 suchthat a plane passing symmetrically through C-shaped hook 56 and throughthe diametrically opposite chevron-shaped projection 58 also passessymmetrically through the bevel at distal end 34 of IV cannula 30.However, in other embodiments collar 50 may be rotatably mounted tohousing 12 so that the orientation of the plane passing symmetricallythrough hook 56 and through projection 58 can be varied relative to aplane passing symmetrically through the bevel at distal end 34 of IVcannula 30.

[0023] Needle assembly 10 further includes a shield 60 with a proximalend 62 and a distal end 64. Portions of shield 60 near proximal end 62include a hinge pin 66 that is snapped into engagement with hook 56 sothat shield 60 can rotate relative to hook 56 from an open positionshown in FIGS. 1 and 2 to a closed position shown in FIG. 5. In the openposition, shield 60 is spaced from IV cannula 30 and is aligned to IVcannula 30 at an obtuse angle of about 120°. The preferred orientationof collar 50 relative to the bevel at distal end 34 of IV cannula 30provides a clear indication of the preferred bevel-up orientation of IVcannula 30 that is employed by most health care technicians during use.Additionally, the orientation of collar 50 on housing 12 ensure thatshield 60 will not interfere with the view of IV cannula 30 duringinsertion. The above-referenced option of a rotatable collar enables theuser to select a preferred orientation of shield 60 relative to thebevel at distal end 34 of IV cannula 30 and relative to the patient andother medical equipment that may be placed in communication with thepatient. Portions of shield 60 near proximal end 62 further include atop wall 68 and sidewalls 70 and 72 that project from top wall 68.Portions of sidewalls 70 and 72 at proximal end 62 of shield 60 arespaced apart by a distance that equals or exceeds the width of outersidewall 18 of housing 12. Thus, sidewalls 70 and 72 easily can berotated on opposite respective sides of outer sidewall 18 as shield 60is rotated within hook 56. The wide spacing of sidewalls 70 and 72 alsofacilitates observations of the flashback chamber 26. Opposed facingsurfaces of sidewalls 70 and 72 at proximal end 62 of shield 60 areformed with collar catches 74 and 76 respectively. Collar catches 74 and76 are configured to snap into engagement with chevron-shaped latch 58on collar 50. Top wall 68 is provided with an array of tactile bumps onthe surface facing away from sidewalls 70 and 72 to provide a visualindication of an actuation landing for a thumb or forefinger and toenhance frictional engagement by the thumb or forefinger when rotatingshield 60.

[0024] Shield 60 narrows at locations between top wall 68 and distal end64 and defines a substantially U-shaped channel of sufficient width toreceive IV cannula 30. As shown most clearly in FIG. 5, channel 78 issufficiently long to encompass all of IV needle 30. Interior portions ofchannel 78 include a resiliently deflectable cannula lock 80 that isangled from one sidewall of channel 78 toward the opposed sidewall andtoward the top wall. Cannula lock 80 will yield in response to contactwith IV cannula 30 as shield 60 is rotated toward IV cannula 30. Cannulalock 80 is further dimensioned to move passed IV cannula 30 when shield60 approaches the closed position. Hence, cannula lock 80 will returnresiliently toward an undeflected position and will trap IV cannula 30within shield 60. Although the figures herein depict a single cannulalock 80, plural cannula locks may be provided. Additionally, cannulalocks may extend from each of the two sidewalls of U-shaped channel 80.Other structures for locking IV cannula 30 in shield 60 can be provided.These include other configurations for plastic or metal cannula locksthat deflect in response to contact with IV cannula 30 and thenresiliently move to trap IV cannula 30. Alternatively, the cannula lockmay require some form of user activation, such as rotation or axialmovement of a portion of the shield.

[0025] Needle assembly 10 further include an IV shield 82 and a NPshield 84 mounted respectively over IV cannula 30 and NP cannula 40 andfrictionally retained on collar 50. Shields 82 and 84 can be separatedby an appropriate application of pulling and/or slight twisting force.

[0026] Needle assembly 10 is used by first separating NP shield 84 fromcollar 50 and then threading distal end 14 of housing 12 into a needleholder 86. Shield 60 then is rotated into the fully open position shownin FIGS. 1 and 2 where shield 60 is spaced from IV cannula 30 and isaligned to IV cannula 30 at an obtuse angle of about 120°. IV shield 78then is removed from its engagement with collar 50. The phlebotomistthen guides distal end 34 of IV cannula 30 into a targeted blood vessel.A pressure differential between the blood in the vein and the pressurewithin housing 12 will cause blood to flow through lumen 36 of IVcannula 30. Blood will appear in flashback chamber 26 very quickly afteraccess to the blood vessel has been attained. This blood can be seen inflashback chamber 26 by the phlebotomist due to the disposition andalignment of shield 60 in the open position. The phlebotomist theninserts an evacuated tube 88 into needle holder 82. Evacuated tube 88includes a rubber stopper 90 that collapses sleeve 46 distally. As aresult, proximal end 42 of NP cannula 40 pierces rubber sleeve 46 andthen pierces stopper 90 of evacuated tube 88. The phlebotomistaccumulates one or more samples of blood in this manner.

[0027] After collecting the last sample of blood, the phlebotomist urgesneedle assembly 10 and needle holder 86 from the patient and immediatelyexerts pressure with a thumb or forefinger on top wall 68 of shield 60.As a result, shield 60 rotates about hook 56 into a closed position, asshown in FIG. 5, so that IV cannula 30 is surrounded by channel 78 ofshield 60. Sufficient rotation causes cannula lock 80 to engage IVcannula 30 and to deflect. Further rotation of shield 60 causes cannulalock 80 to pass IV cannula 30. As a result, cannula lock 80 resilientlyreturns toward and undeflected condition and traps IV cannula 30.Simultaneously, collar catches 74 and 76 snap into engagement withchevron-shaped latch 58 on collar 50 to provide an audible and tactileindication of shielding. The safely shielded needle assembly then can bediscarded in a sharps receptacle.

[0028] The needle assembly provides effective shielding without impedinguse of needle assembly 10 and without obscuring observation of flashbackchamber 26. The high visibility of flashback chamber 26 is partlyattributable to the extreme distal position of the space between IVcannula 30 and NP cannula 40 and the corresponding proximal position ofshield 60 in the open position. Additionally, as shown most clearly inFIG. 1, proximal end 62 of shield 60 is widely open between sidewalls 70and 72 to provide a broad range of acceptable viewing angles.

[0029] The above-described needle assembly has hinge pin 66 of shield 60snapped into engagement with C-shaped hook 56 on collar 50 for hingedrotation of shield 60 relative to collar 50 and relative to housing 12.However, other structures for accommodating hinged movement of shield 60relative to collar 50 and/or housing 12 can be provided. These includeforming shield 60 unitarily with collar 50 so that a living hingedconnection between shield 60 and collar 50 is provided. A shield with aliving hinge is shown, for example, in U.S. Pat. No. 5,681,295. Suchhinged connections can also include an over-center hinge where theshield is substantially unbiased in the fully open position and in thefully closed position. However, the user may have to overcome a bias inthe over-center hinge during the initial phases of rotation from theopen position to the closed position. The over-center hinge thenresiliently returns toward an unbiased condition and accelerates thehinge towards the closed position.

What is claimed is:
 1. A needle assembly comprising: a housing havingproximal and distal ends and a chamber wall extending between said endsfor defining a chamber in said housing, at least a portion of saidchamber wall being formed from a material that permits observation of afluid flowing into said chamber, and an IV cannula projecting distallyfrom said housing and communicating with said chamber, a shield hingedto said housing and being positionable in an open position where saidshield is spaced from said IV cannula and aligned to permit observationof said chamber, said shield further being rotatable to a closedposition where said shield covers said IV cannula.
 2. The needleassembly of claim 11 wherein at least a portion of said chamber isdisposed distally of a location at which said shield is hingedlyconnected to said housing.
 3. The needle assembly of claim 2, whereinsaid IV cannula communicates with a portion of said chamber adjacentsaid distal end of said housing, said shield being hingedly connected toa portion of said housing substantially adjacent said proximal end ofsaid housing.
 4. The needle assembly of claim 1, wherein said shieldincludes at least one lock for locked engagement with said IV cannulawhen said shield is in said closed position.
 5. The needle assembly ofclaim 4, wherein said shield comprises at least one catch for engaging aportion of said housing spaced from said hinged connection when saidshield is in said closed position.
 6. The needle assembly of claim 5,wherein said at least one catch on said shield for engaging said housingis configured for generating audible and tactile indications ofshielding.
 7. The needle assembly of claim 1, further comprising anon-patient cannula projecting from said housing and communicating withsaid chamber.
 8. The needle assembly of claim 7, wherein saidnon-patient cannula includes a distal end disposed in said chamber, saiddistal end of said non-patient cannula being disposed distally of thehinged connection of said shield to said housing.
 9. The needle assemblyof claim 8, wherein said non-patient cannula extends sufficientlythrough said housing such that said distal end of said non-patientcannula is substantially adjacent said distal end of said housing, saiddistal end of said non-patient cannula being substantially aligned withand spaced slightly from said IV cannula.
 10. The needle assembly ofclaim 1, wherein said chamber wall is an outer chamber wall, saidhousing further comprising an inner chamber wall defining a flashbackchamber within said housing, said inner chamber wall extending from saiddistal end of said housing toward said proximal end of said housing suchthat said IV cannula communicates with said flashback chamber defined bysaid inner chamber wall.
 11. The needle assembly of claim 10, furthercomprising a non-patient cannula mounted to said proximal end of saidhousing, said non-patient cannula having a proximal end projectingproximally from said housing and a distal end projecting into saidflashback chamber.
 12. The needle assembly of claim 1, wherein said IVcannula has a distal end with a substantially planar bevel defining atip, said shield being hinged for movement through a plane substantiallycoincident with a plane of symmetry passing through said bevel at saiddistal end of said IV cannula.
 13. The needle assembly of claim 1,wherein said housing comprises a collar mounted around a portion of saidhousing, said shield being hinged to said collar.
 14. The needleassembly of claim 13, wherein said collar is fixed to said housing. 15.The needle assembly of claim 13, wherein said housing includes a centralaxis extending between said proximal and distal ends, said collar beingrotatable around said central axis for orienting said shield at apreferred rotational alignment relative to said IV cannula.
 16. Theneedle assembly of claim 13, wherein said collar is mounted to saidhousing at a location proximally of said chamber.
 17. The needleassembly of claim 13, wherein said shield is unitary with said collar.18. A shieldable needle assembly comprising: a housing having a proximalend wall, a distal end wall and an external sidewall extending betweensaid proximal and distal end walls, said proximal and distal end wallsand said external sidewall defining a chamber within said housing, atleast a portion of said external sidewall being formed from asubstantially transparent material for permitting visual observation ofsaid chamber, an IV cannula having a proximal end mounted in said distalend wall of said housing, a distal end externally of said housing and alumen extending between said proximal and distal ends of said IV cannulafor communicating with said chamber, a non-patient cannula havingopposite proximal and distal ends and a lumen extending between saidends, said distal end of said non-patient cannula being disposed in saidchamber and said proximal end of said non-patient cannula being disposedexternally of said chamber, a shield pivotally mounted to said housingat a location distally of said proximal end of said IV cannula, saidshield being rotatable from an open position where said IV cannula isexposed for use and a closed position where said shield covers said IVcannula for preventing accidental sticks with said IV cannula, wherebysaid shield in said open position permits visual observation of portionsof said chamber adjacent said proximal end wall of said housing.
 19. Theshieldable needle assembly of claim 18, wherein said shield is hinged toa location substantially adjacent said proximal end wall of saidhousing.
 20. The shieldable needle assembly of claim 18, wherein saidshield, in said open position, is aligned at a rotational positionspaced more than 90° from said IV cannula.
 21. The shieldable needleassembly of claim 20, further comprising at least one lock on saidshield for locking said shield in said closed position.